Change default theme to "system_serif"

http/request/context.go

@@ -64,7 +64,7 @@ func UserLanguage(r *http.Request) string {
 func UserTheme(r *http.Request) string {
 	theme := getContextStringValue(r, UserThemeContextKey)
 	if theme == "" {
-		theme = "light_serif"
+		theme = "system_serif"
 	}
 	return theme
 }

http/request/context_test.go

@@ -241,7 +241,7 @@ func TestUserTheme(t *testing.T) {
 	r, _ := http.NewRequest("GET", "http://example.org", nil)
 
 	result := UserTheme(r)
-	expected := "light_serif"
+	expected := "system_serif"
 
 	if result != expected {
 		t.Errorf(`Unexpected context value, got %q instead of %q`, result, expected)